Onsite troubleshooting: 1. Actual measurement revealed that optical power on both ends was -20db or so, which met the requirements. Therefore, the possibility of a faulty module or fiber was excluded. 2. Careful observation of the fiber and the module indicated that the head of the fiber was polluted. Since the head of the fiber was cleaned, the link has not been abnormal so far. Therefore, it could be decided that the polluted head of the fiber caused the receiving end to generate error codes and then the interface to be abnormal.

Root Cause

The abnormal link causes the interface to be abnormally Up/Down. The board itself is not faulty. Theoretical analysis: 1. View the interface count and a CRC error existed on both ends. CRC found check errors when FRAMER resolved data. Possible reasons are as follows: (1) Data interfered during the transmission on the link led to error codes. (2) Optical power was not stable. (3) The sending interface of the peer was faulty. (4) Resolution by the local end was incorrect. 2. Analyze log information. It was found that the time when the Up/Down fault occurred on the two ends differed. When configured with the negotiation function, an interface initiates a negotiation frame automatically after feeling optical receiving. If both ends receive the negotiation frame, the interfaces of both ends are Up. The inconsistency in the time when the Up/Down fault occurred on the two ends showed that one end received the negotiation frame and the other end did not. This was probably because errors occurred during the transmission of packets on the link. With the results from analysis combined, the link was confirmed abnormal.

Suggestions

For long-distance transmission applications in actual use, you must ensure the fiber to be curved within 90 degrees and prevent the fiber from being damaged, polluted, or badly plugged.